The present invention relates generally to spray formed components, and more particularly to spray formed components having properties comparable to those of corresponding forged components.
Forging has long been used to produce components for demanding applications, e.g., for components which require a combination of high strength and other desired properties such as low crack growth rates and high stress rupture resistance. In the aerospace industry, forging is used to produce parts having complex shapes such as blades and vanes, and annular-shaped components such as engine cases, flanges and seals, each of which typically requires a combination of high strength, low crack growth rates and high stress rupture resistance.
With particular reference to forging annular-shaped components, a billet of material is obtained having a composition corresponding to the desired composition of the finished component. The billet is typically prepared from ingots of the material. The billet is first pierced, and is then thermomechanically processed, such as by ring-rolling one or more times to transform the billet material into the general component shape. The component may also be heat treated to obtain desired properties, e.g., a particular level of fatigue crack growth resistance, and then finished, e.g., polished or machined to provide the component with the precise dimensions or features.
The production of components by forging is an expensive, time consuming process, and thus is typically warranted only for components that require a particularly high level of various properties, e.g., high strength with low crack growth rates and high stress rupture resistance. With respect to obtaining the billets for forging, certain materials require lead times measured in months. During component fabrication, much of the original billet material is removed and does not form part of the finished component, e.g., it is waste. The complexity of the shape of the component produced merely adds to the effort and expense required to fabricate the component. In addition, finished components may still require extensive machining or other finishing. Moreover, in order to operate gas turbine engines at higher temperatures to increase efficiency or power or both, components fabricated from increasingly more advanced alloys are required. Many of these more advanced alloys are increasingly difficult or impossible to forge, which adds further to the cost of the components or renders the components so expensive that it is not economically feasible to exploit certain advances in engine technology, or to utilize particular alloys for some components.
Spray forming has not previously been used to produce components directly from bulk material, e.g., material in ingot form, which exhibit not only high strength, but also low crack growth rates and high stress rupture resistance. In the case of IN 718, discussed further below including reference to FIG. 5, low crack growth rates and high stress rupture resistance corresponds to meeting the requirements set forth in Aerospace Material Specification AMS 5663 (Rev. H, publ. January 1996), published by SAE Int'l of Warrendale, Pa., and is incorporated by reference herein. It is this combination which is produced in accordance with the present invention. A typical spray forming apparatus is illustrated in FIG. 1. Metal is provided in ingot form and melted in a crucible 12, preferably in a vacuum melt chamber 14 at low pressure and/or in a non-reactive environment. The molten metal 16 is transferred to a tundish 18, and then passes through an atomizer 20, which utilizes an inert carrier gas such as argon to entrain atomized metal droplets. The atomized material 22 impinges upon and is deposited onto a cooled mandrel or substrate 24 that is located in a spray chamber 26. In order to form an annular component, the mandrel is cylindrical and may be rotated, and the stream of atomized metal and the mandrel may be scanned relative to one another. The metal impinges upon the substrate and previously deposited metal, and solidifies rapidly. Layers of the solidified metal then build upon one another to form the desired article. See, e.g., U.S. Pat. No. 4,830,084. The article may then be further treated, e.g., by hot isostatic pressing (HIP'ing) and/or thermomechanically processing such as by ring rolling to densify and strengthen the material. Superalloys have been melted and spray formed in this manner to form parts, although such parts as formed lack properties such as high strength. Low crack growth rates or stress rupture resistance and thus cannot be employed as formed in demanding applications such as gas turbine engines or other high temperature and pressure environments.
One material which has been widely employed in producing forged parts for use in demanding applications is Inconel 718 ("IN 718"), which has a nominal composition of about 19 w/o Cr, 3.1 w/o Mo, 5.3 w/o Cb+Ta, 0.9 w/0 Ti, 0.6 w/o Al, 19 w/o Fe, balance essentially nickel and nominal amounts (in weight percentage) of other elements. As noted above, exemplary parts include gas turbine engine cases, flanges and seals, as well as blades and vanes. Once formed, these parts typically must still be machined and heat treated to obtain desired properties. AMS 5663 is a conventional heat treatment for parts forged from IN 718 and is incorporated by reference herein.
Under AMS 5663, a forged component is heat treated in two steps. The first step includes a solution heat treatment at a temperature of between 1725-1850.degree. F., for a time that is proportional to the cross sectional thickness of the component, and then cooling at a rate equivalent to air cooling or faster. The second step includes a precipitation heat treatment at a temperature of between 1325-1400.degree. F. for about eight (8) hours, followed by cooling at a rate of about 100.degree. F. per hour to a temperature of about 1150-1200.degree. F. and held at that temperature for about eight (8) hours, and then air cooled. The precipitation heat treatment may be altered by furnace cooling the part from 1325-1400.degree. F. to 1150-1200.degree. F. at any rate so long as the overall precipitation heat treatment time is about eighteen (18) hours. The resulting parts have yield strengths of at least about 150 ksi at room temperature and at least about 125 ksi at 1200.degree. F., and exhibit relatively low notch sensitivity and high stress rupture resistance. Accordingly, parts produced by forging IN 718 and heat treated in accordance with AMS 5663 are suitable for use as gas turbine engine cases, flanges or seals, blades and vanes, as well as other demanding applications. However, forged components also often exhibit significant levels of coarse carbides and other inclusions, the levels of which vary significantly from component to component. Forged components tend to be difficult to machine and inspect. Moreover, precise reproducibility is also a concern--forging does not always result in components having dimensions that are identical from part to part. After inspection, many parts must still be re-worked. As a general rule, it is believed that forged parts must be scrapped or re-worked about 20% of the time.
In an effort to produce components more repeatably and at less expense, parts have been spray formed using IN 718. As spray formed and HIP'd, these parts do have significant strength, but exhibit high crack growth rates and inferior stress rupture resistance, and it has been believed that such parts need to be thermomechanically processed, e.g., forged or ring-rolled, to obtain these properties. The expense of such an added step has not been attractive.
As noted above, a standard, conventional heat treatment for components forged from IN 718 is set out in AMS 5663. However, we have determined that parts sprayformed from IN 718, and then HIP'ed and heat treated in accordance with AMS 5663 or other conventional heat treatments exhibit yield and tensile strengths similar to forged, but exhibit such inferior crack growth rates and stress rupture resistance that the components cannot be used in demanding applications when these considerations must be addressed.
It is a general object of the present invention to provide spray formed articles having properties comparable to properties of corresponding forged articles.
It is more specific object of the present invention to provide spray formed articles having a balance of strength, crack growth rates and stress rupture resistance comparable to corresponding forged articles.
It is another object of the present invention to provide a heat treatment for spray formed articles, whereby crack growth rates of the articles are low and stress rupture resistance of the articles are high.
It is yet another object of the present invention to provide a heat treatment to enable spray forming of materials which are not amenable to fabrication by conventional forging techniques.
It is still another object of the present invention to provide such a heat treatment to provide articles composed of spray formed IN 718 with properties comparable to those of corresponding articles forged from IN 718.